1. Field of the Invention
The present invention relates to a magnetic head drive control device of a magnetic recording/reproducing apparatus using a servo scheme.
2. Description of the Related Art
Conventionally, a hard disk drive (HDD), for example, comprises a magnetic head drive control device for positioning a magnetic head at a target position (target track) of a recording medium by using servo data recorded beforehand on a servo surface of the recording medium. More specifically, this device includes a position signal decoder 10 as shown in FIG. 1. The decoder 10 receives a servo signal read out from a servo surface 11a of a recording medium 11 and amplified and supplied by a head amplifier 13.
A magnetic head of an HDD is composed of a servo head 12 and a plurality of data heads 14 corresponding to a plurality of data surfaces except for the servo surface 11a. The heads 12 and 14 are arranged to seek in the radial direction of the recording medium 11 by an actuator 15. The actuator 15 is constituted by a supporting mechanism 15a for supporting the heads 12 and 14, a rotary shaft 15b and a voice coil motor (VCM) 15c. The supporting mechanism 15a is driven to rotate around the shaft 15b by a driving force of the VCM motor 15c. The recording medium 11 is rotated by a spindle motor 16.
The position signal decoder 10 generates a position error signal PE corresponding to a current position of the servo head 12. On the basis of a change in signal PE output from the decoder 10, a track counter 17 generates a track pulse TP each time the head 12 crosses a track. A velocity signal generator 18 differentiates the position error signal PE and generates a velocity signal VR corresponding to an actual moving velocity of the head 12. A compensation circuit 19 stabilizes a servo loop when a positioning control mode is executed on the basis of the position error signal PE.
Drive control is roughly classified into a velocity control mode and a positioning control mode. The positioning control mode is roughly classified into a transient control mode and a tracking control mode. In the velocity control mode, a microprocessor (CPU) 20 counts track pulses TP from the track counter 17 and calculates a distance to a target position to which the head 12 is to be moved. The CPU 20 refers to a preformed table representing a correspondence between a distance and an optimal target velocity and outputs target velocity data corresponding to the calculated distance. A target velocity generator 21 outputs a target velocity signal VT corresponding to the target velocity data from the CPU 20 to a mixing switching unit 22. Under the control of the CPU 20, the unit 22 outputs an error signal ES indicating a difference between the velocity signal VR and the target velocity signal VT to a VCM driver 24 via a mechanical filter 23 in the velocity control mode. The VCM driver 24 supplies a drive current corresponding to the error signal ES to the VCM 15c to drive the actuator 15. The CPU 20 exchanges various control signals and data with respect to a disk controller (HDC).
In the velocity control mode, as shown in FIGS. 2A through 2D, a control region is roughly classified into acceleration and deceleration regions. In the acceleration region, the error signal ES substantially reaches its maximum value (saturation level) since the error between the velocity signal VR and the target velocity signal VT is large. Therefore, a drive current corresponding to the error signal ES is supplied to the VCM 15c to accelerate the actuator 15. The CPU 20 updates the target velocity data each time the track pulse TP is output. As the head 12 moves closer to the target position, an absolute value of the target velocity signal VT is decreased. From a timing T1 at which the velocity signal VR reaches the target velocity signal VT, the actuator 15 is switched to deceleration on the basis of the decreased target velocity signal TV.
From a timing T2 at which the velocity of the head 12 becomes substantially 0, the velocity control mode is switched to the positioning control mode. A switching timing from the velocity control mode to the positioning control mode is determined by the CPU 20 by determining that a difference between the value of the track pulse TP from the track counter 17 and the target track becomes, e.g., one track.
In the positioning control mode, the mixing switching unit 22 outputs an output from the compensation circuit 19 to the VCM driver 24 as the signal ES. As a result, the head 12 is positioned (settled) at the center of the target track. The data head 14 is positioned at the center of the target track following the servo head 12. The CPU 20 switches the characteristics (e.g., an advancing amount of a phase) of the compensation circuit 19 to obtain optimal characteristics for each of the transient and tracking control modes.
The velocity control mode is a servo system as shown in FIG. 3. A predetermined voltage is supplied to the VCM driver 24. The driver 24 outputs a current proportional to the supplied voltage to the actuator 15. As a result, the actuator 15 operates at a certain velocity. The actual velocity signal VR generated from the servo signal by the position signal decoder 10 and the velocity signal generator 18 is compared with the target velocity signal VT, and a voltage proportional to the difference is supplied to the VCM driver 24. As a result, a current proportional to the difference is supplied to the actuator 15. Such a control operation is repeatedly performed to change the actual velocity closer to the target velocity.
The positioning control mode is a servo system as shown in FIG. 4. Difference data (indicating whether the magnetic head is located on a track or offset outside or inside from the center of the track) between actuator position information X from the actuator 15 and a target position R is supplied to the position signal decoder 10. The decoder 10 decodes the difference data and outputs the data to the position error signal compensation circuit 19. The circuit 19 advances a phase to compensate for a delay produced by a secondary integral calculation when a position is to be controlled by a current.
Generally, in a servo system, by increasing a loop gain in consideration of the instability of the system, a tracking error can be reduced to improve a response characteristic. Actually, however, a frequency band of a sensor system (e.g., the head amplifier 13 and the position signal decoder 10) is limited. In addition, the influence of a resonant point of a mechanism system cannot be ignored. Therefore, if the gain is increased too high in order to shorten a settling time, the system becomes unstable. Therefore, since the gain must be adjusted to be a proper value, an essential index for determining the performance of the servo system such as a tracking performance or a response speed cannot be improved better than a specific predetermined limit.
An analog servo system is generally expressed by a block diagram as shown in FIG. 5. In this analog servo system, a relationship between an input (target value R) and an output (result C) is Laplace-transformed as represented by equation (1): EQU C=G(s).multidot.R(s)/(1G(s)H(s) (1)
where G, H and R are the rational polynomials of s in a linear system. Therefore, equation (1) can be rewritten as the following equation (2) by using N and D (rational polynomials of s): EQU C=N(s)/D(s) (2)
Equation (2) can be expressed as follows by using s: ##EQU1##
Equation (3) can be developed into partial fractions, and a time response c(t) of C(s) is obtained by equation (4) by Laplace-transforming the partial fractions: EQU C(t)=.SIGMA.Aq*exp(-.delta.qt)+.SIGMA.Br*(1/.omega.r)*exp(-.alpha.rt)*sin(. omega.rt) (4)
for .delta.q, .alpha.r and .omega.r&gt;0.
In equation (4), exp(-.delta.t)(.delta.&gt;0) (i.e., exp(-.delta.qt) and exp(.delta.rt)) is always included in each term of c(t). Therefore, all terms converge to 0 for t.fwdarw..infin. but a predetermined value always remains for a finite time.
As described above, since a head drive control device using an analog closed servo control scheme depends on the performance of a servo system, improvements in performance are limited. More specifically, a settling time of the head in the positioning control mode cannot be reduced to be shorter than a certain predetermined period of time.